The present invention relates to an electric double layer capacitor of high electrostatic capacitance which utilizes the principles of an electric double layer that is composed of an interface between activated carbon and electrolyte.
Recently, high-capacitance capacitors based on the electric double layer principles have been developed as backup power supplies for memories in electronic systems, and are widely used with microcomputers and IC memories.
One type of electric double layer capacitor is disclosed in U.S. Pat. No. 3,536,963, for example. Disclosed electric double layer capacitor comprises a frame-shaped gasket, a pair of polarized electrodes made of activated carbon particles and disposed in the gasket, a separator disposed between the polarized electrodes for preventing electrons from moving between the electrodes, and a pair of current collectors mounted respectively on the opposite surfaces of the polarized electrodes and having peripheral edges bonded to the gasket. The polarized electrodes are made as paste electrodes from a concentrated slurry which is a mixture of powdery or particulate activated carbon and an electrolyte.
It is important that the internal resistance of such an electric double layer capacitor with paste electrodes be low. The internal resistance of an electric double layer capacitor is greatly affected by the contact resistance of active carbon of the polarized electrodes and the contact resistance between the collector electrodes and the polarized electrodes.
Therefore, in order to reduce the internal resistance of the polarized electrodes and the contact resistance between the collector and polarized electrodes, each basic cell of the electric double layer capacitor should be kept under vertical pressure to bring the particles of the paste activated carbon into good electric contact with each other. Conventional electric double layer capacitors require each cell to be kept under a pressure of about 100 kg/cm.sup.2 though it depends on the size of the electrodes, the size of the particles of the carbon material, or the kind of the electrolyte used. In prior electric double layer capacitors, the cells are kept under pressure by deforming the outer cases of the capacitors or bonding the current collectors strongly to gaskets. If an electric double layer capacitor is to be used as a large-capacitance capacitor, e.g., a power supply for energizing a motor, then it is necessary to increase the cross-sectional areas of the electrodes of the basis cell. Therefore, the pressure to be applied to the basic cell has to be increased. Increasing the pressure, however, causes some practical problems such as the selection of means for applying the pressure and the need for high rigidity for the outer cell which houses the basic cell.
In order to solve the problems of the polarized electrodes in the form of paste electrodes, there has been proposed an electric double layer capacitor which employs an electrode body composed of a sintered mixture of carbon powder having a large specific surface area and metal powder that is inert with respect to an electrolytic solution, as disclosed in Japanese Laid-Open Patent Publication No. 54(1979)-24100. The disclosed electric double layer capacitor includes a lead connected to the sintered electrode which is housed in a casing and extending through the wall of the casing for drawing electric charges stored in the sintered electrode. Ordinary electric double layer capacitors are designed to draw electric charges stored in a polarized electrode from current collectors that are held in contact with upper and lower surfaces of the polarized electrode. Attempts have been made to minimize the contact resistance between the polarized electrode and the current collectors.
Generally, a sintered polarized electrode and a current collector can be joined to each other in different ways as shown in FIGS. 3(a), 3(b), and 3(c) of the accompanying drawings. According to the process shown in FIG. 3(a), a current collector which comprises an electrically conductive film is simply held in contact with a polarized electrode. However, since the contact resistance between the current collector and the polarized electrode is large and the current collector and the polarized electrode are not in stable contact with each other, the resultant capacitor has a high internal resistance. The joining method shown in FIG. 3(b) uses an electrically conductive adhesive by which a current collector in the form of an electrically conductive film is joined to a polarized electrode. The contact achieved by this method is stabler than the contact established by the process shown in FIG 3(a). The internal resistance of the resultant capacitor cannot however be reduced beyond a certain limit because the adhesive layer is interposed at the interface between the polarized electrode and the current collector. FIG. 3(c) shows a joining arrangement in which a current collector in the form of an electrically conductive film is joined to a polarized electrode under pressure with heat. While a smaller contact resistance is achieved between the polarized electrode and the current collector by this joining process, it takes about 10 minutes to keep the current collector and the polarized electrode under pressure with heat until they are joined together. Furthermore, unless the current collector and the polarized electrode are uniformly pressed against each other, the current collector are wrinkled and ruptured.